Today I had a weird problem with my email, and just recently found a solution.

On my web application and command line scripts on the server, email refused to send. Ruby's net/smtp returned a 451 error. I played around a bit, and found that it was due to the fact the account I was listing as the sender was not a valid account AND I had no catchall that matched the account.

I was trying to send email from do-not-reply@ domain.com, and it just seemed broken. do-not-reply was not a real account.